My last article (about concrete mortar flaking) generated more comments than any I’ve written to date. In case I left any doubt, I believe that the new “hard” rock specification used by the Kansas City Metro Materials Board (http://www.kcmmb.org) will produce better and more durable concrete than is possible with the currently available local limestone aggregates, PERIOD. It obviously costs more to import coarse aggregate, but that initial cost increase is modest, especially when factored into the cost of the end product (which will include labor and which may include steel and/or formwork). I also believe that the life cycle costs will be lower as a result of the increased longevity and first costs are much lower than subsequent removal and replacement costs. I’m referring here to outside concrete subjected to wetting, freezing and thawing. For interior concrete, limestone aggregate is arguably a better choice. It’s less expensive and has a lower coefficient of thermal expansion.
That being said, changing our aggregate sources will not solve all of the problems we face when using concrete as a building material.
I looked at a RCB project that developed a number of cracks recently. The concrete placement began at 6:00 AM on July 18th (which is at the far left on the following charts). I used http://www.stormwatch.com to obtain weather information near the construction site. This combination of air temperature and humidity is risky regardless of wind speed. PCA publishes a nomograph in Design and Control of Concrete Mixtures that relates air temp, concrete temp, relative humidity and wind speed to evaporation rates. This reference was based on the work of Carl Menzel and was first published in 1954.
There are several concrete evaporation reducers that are available through at least some of the ready-mix suppliers in the metro area. These products are applied to the concrete surface immediately after floating (and before final brooming) to reduce the harmful effects of evaporation. They do NOT replace any part of the curing process. If you absolutely must place concrete in the kind of conditions pictured above, then I’d consider using of one of these products.
One of the products of FHWA’s SHRP research is HIPERPAV, now at version 2.5. This software allows the user model “the early-age development of concrete strength and stresses that result from moisture and temperature changes within the pavement.” It considers many more factors than the Menzel model, and provides a plot of stress and strength vs. time (or time of day). Here’s an example of its output.
HIPERPAV is available for download at http://www.hiperpav.com. It’s free for 21 days, and then less than $500 if you want support, less then $200 if you don’t.
I’d like to end the practice of land-filling brand new cracked concrete, especially the expensive “hard rock” variety.
